Apparatus for extracting power from fluid flow

ABSTRACT

An apparatus for extracting power includes a track and an airfoil coupled to the track. The track includes first and second elongate sections, where the first elongate section is positioned above the second elongate section. The airfoil includes a pressure surface lying between a suction surface and the track, and is moveable in opposite directions when alternately coupled to the first elongate section and second elongate section.

FIELD OF THE DISCLOSURE

This disclosure generally relates to renewable energy. Morespecifically, this disclosure describes apparatuses and methods forextracting power from fluid flow.

BACKGROUND OF THE INVENTION

Extracting power from fluid flow is a prominent source of renewableenergy. Mainstream examples include wind power and hydropower.

Traditional systems for extracting power from fluid flow are primarilyturbine-based. In a turbine, one or more blades are rotatable about acentral point, which is rigidly attached to an anchor (typically atower). The blades are placed within the flowing fluid, which induces arotation of the blades, and the rotation is converted to electricity.

Turbines may suffer from a number of drawbacks. For example, the forcesexerted on a turbine are proportional to the cube of the length of theturbine blades. As the turbine blades increase in size, destructiveforces (the moment about a tower, for example) are cubed. By contrast,usable power is only squared.

This “square-cube” law places significant restrictions on the scale ofturbines. Inevitably, the gain of additional power extracted fromgreater size is not offset by the cost of addressing an increase indestructive forces. For at least this reason, turbine scale is limited.

Other known solutions eliminate towers or other rigid anchors. Examplesof such power extraction systems include airborne wind energy systems(“AWE”). Typically, these systems are aerodynamic bodies tethered to theground (a kite, for example) which fly at altitudes above the height ofwind turbines.

There are two main mechanisms for extracting power from an AWE'smovement through air: on-board power generation and ground-based powergeneration. An example of the former includes a turbine on the kitewhich generates electricity in the same way as the turbines discussedabove. An example of the latter includes a long tether attached to adrum, where movement of the kite unrolls the tether from the drum, whichrotates the drum and a connected generator, thus converting wind powerinto electricity.

AWEs may also suffer from a number of drawbacks. For example, becausethe system requires a tether angled to the airborne object, the powerextracted will be a function of the available power and the cosine ofthe tether angle. Thus, the power extracted may never equal theavailable power. In addition, the tether will create drag as it movesthrough the air, slowing the kite, and thus reducing the harvestedpower. Finally, high-flying AWEs are subject to aviation restrictions,which limit their geographic scope (due to no-fly zones, for example)and present regulatory hurdles for implementation.

SUMMARY

Examples of the disclosure are directed toward apparatuses and methodsfor extracting power from fluid flow that overcome the above-identifieddrawbacks. As an exemplary advantage, the scale of the apparatuses maynot be limited by a square-cubed law. As another exemplary advantage,the apparatuses and methods may not be subject to tether drag and/orcosine losses. As another exemplary advantage, the apparatuses may notbe classified as “airborne devices” for regulatory purposes.

In some examples, an apparatus for extracting power includes a track andan airfoil coupled to the track. The track includes first and secondelongate sections, where the first elongate section is positioned abovethe second elongate section. The airfoil includes a pressure surfacepositioned between a suction surface and the track, and the airfoil ismoveable in opposite directions when alternately coupled to the firstelongate section and second elongate section.

By facing the suction surface toward the track, the track may beoriented so that the airfoil moves crosswind with respect to anatmospheric wind speed. This crosswind motion may advantageously allowthe airfoil to travel at speeds greater than the speed of theatmospheric wind. Further, by positioning the first elongate sectionabove the second elongate section, an airfoil traveling on either of thesections will directly receive the atmospheric wind; that is, theincident wind on an airfoil is not disturbed by airfoils on the otherelongate section. This may allow for increased power extraction.

In some further examples, a bridle is coupled to the track and anchoredto the ground. Bridling may beneficially allow for less structuralsupport, reducing the cost of the power extraction apparatus.

Bridling may also beneficially allow for reductions in destructiveforces on the apparatus. For example, three or more bridles may bedistributed along an elongate section to reduce the moment on a lengthof the elongate section.

In some further examples, an airfoil is rolled at approximately 90−γdegrees to the horizon to offset forces from a bridle angle at γ degreesto the horizon.

In some examples, an airfoil has a first roll to the horizon when theairfoil is coupled to the first elongate section and a second, differentroll to the horizon when the airfoil is coupled to the second elongatesection.

In some examples, the track includes a terminal connecting the first andsecond elongate sections, where the airfoil decelerates when the airfoiltransitions from the first elongate section to the terminal andaccelerates when the airfoil transitions from the terminal to the secondelongate section. This may eliminate high forces attendant with thechange in direction. The deceleration of the airfoil may also beharvested as power.

In some examples, the track includes a terminal connecting the first andsecond elongate sections, wherein the airfoil is yawed as the airfoiltravels along the terminal.

In some examples of the disclosure, a method of extracting powerincludes providing a track, positioning the track, coupling an airframeto the track, and harvesting power from an atmospheric wind through themovement of the airframe. As used herein, an airframe may include anairfoil, and may also include a fuselage and empennage, for example. Anairframe may also simply be an airfoil. The track may include a firstelongate section and a second elongate section lower than the firstelongate section. The track may be positioned so that the airframetravels crosswind to an atmospheric wind.

Some further examples include attaching a bridle to the track andanchoring the bridle. Yet further examples include attaching at leastthree bridles to the first elongate section. In some examples, one ofthe bridles is angled at γ degrees to a horizon and the method includesrolling the airframe at approximately 90−γ degrees to the horizon whencoupled to the first elongate section.

In some examples, the method includes rolling the airframe at a firstangle to a horizon when the airframe is coupled to the first elongatesection and rolling the airframe at a second angle to the horizon whenthe airframe is coupled to the second elongate section, where the firstangle is different from the second angle.

Some examples include coupling a terminal between the first and secondelongate sections, where the airframe decelerates when the airframetransitions from the first elongate section to the terminal andaccelerates when the airframe transitions from the terminal to thesecond elongate section.

In some examples, the method includes coupling a terminal between thefirst and second elongate sections and yawing the airframe as theairframe travels along the terminal.

In some examples of the disclosure, a power extraction system includes atrack, a bridle angled at γ degrees to a horizon and coupled to thetrack, and an airframe coupled to the track and rolled at approximately90−γ degrees to the horizon.

In some examples of the disclosure, a method of extracting powerincludes providing a track, coupling an airframe to the track,positioning the track so that the airframe travels crosswind to anatmospheric wind, attaching a bridle to the track and anchoring thebridle so that it is angled at γ degrees to a horizon, rolling theairframe at approximately 90−γ degrees to the horizon, and harvestingpower from the atmospheric wind through the movement of the airframe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate an exemplary power extraction apparatusaccording to examples of the disclosure. FIG. 1A illustrates theapparatus viewed in a direction of flow of an atmospheric wind. FIG. 1Billustrates the apparatus in a side, cut-away view.

FIG. 2A illustrates an exemplary airframe according to examples of thedisclosure.

FIG. 2B illustrates a cross-section of an exemplary airfoil according toexamples of the disclosure.

FIG. 2C illustrates an exemplary relative wind velocity in accordancewith one example of an object traveling crosswind.

FIGS. 3A and 3B illustrate an exemplary bridling system according toexamples of the disclosure. FIG. 3A illustrates a side view of thebridling system and FIG. 3B illustrates a top view.

FIG. 3C illustrates the forces on a power extraction system when anairframe is rolled 90 degrees to the horizontal according to examples ofthe disclosure.

FIG. 3D illustrates a side view of an exemplary roll introduced on anairframe according to examples of the disclosure.

FIG. 4 illustrates a method of extracting power according to examples ofthe disclosure.

DETAILED DESCRIPTION

In the following description of embodiments, reference is made to theaccompanying drawings which form a part hereof, and in which it is shownby way of illustration specific embodiments which can be practiced. Itis to be understood that other embodiments can be used and structuralchanges can be made without departing from the scope of the disclosedembodiments.

Examples of the disclosure are apparatuses that include a track and anairfoil coupled to the track. The track includes first and secondelongate sections, where the first elongate section is positioned abovethe second elongate section. The airfoil includes a pressure surfacelying between a suction surface and the track, and is moveable inopposite directions when alternately coupled to the first elongatesection and second elongate section.

In some examples, methods of extracting power include providing a track,positioning the track, coupling an airframe to the track, and harvestingpower from an atmospheric wind through the movement of the airframe. Thetrack may include a first elongate section and a second elongate sectionlower than the first elongate section. The track may be positioned sothat the airframe travels crosswind to the atmospheric wind.

FIGS. 1A and 1B illustrate an exemplary apparatus 100 for extractingpower according to examples of the disclosure. FIG. 1A illustrates theapparatus viewed in a direction of flow of an atmospheric wind 124. FIG.1B illustrates the apparatus in a side, cut-away view from the dashedline in FIG. 1A and looking toward end 108.

Apparatus 100 includes airframes 112 and 116 traveling on an upperelongate section 104 and a lower elongate section 106, respectively.Elongate sections 104 and 106 are components of track 102, which alsoincludes terminals 108 and 110.

Airframes 112 and 116 are coupled to track 102 through carriers 114 and118. The tracks are oriented so that the airframes travel crosswind withrespect to the atmospheric wind 124. As used herein, an object may beunderstood to be traveling “crosswind” when the object's direction oftravel is not aligned with a direction of an atmospheric wind. Theatmospheric wind may be a prevailing wind, but need not be so limited.

In some examples, an object travels crosswind when its direction oftravel is perpendicular to a direction of an atmospheric wind. In someexamples, an object travels crosswind when its direction of travel isless than +/−45 degrees from a direction that is perpendicular to adirection of an atmospheric wind. In some examples, an object travelscrosswind when its direction of travel is less than +/−20 degrees from adirection perpendicular to a direction of travel of an atmospheric wind.

In some examples, a maximum degree offset from a perpendicular directionof travel (in relation to the atmospheric wind) may be calculated by alimit on efficiency losses. In some further examples, an algorithm maybe used to relate the degree offset (β) to the limit on efficiency loss(EL). Exemplary algorithms may include EL=(1−cos β) and EL=(1−cos² β).For a given limit on efficiency loss, the maximum offset angle can bedetermined.

By traveling crosswind, some examples described herein may be capable oftraveling many times the atmospheric wind-speed. Additionally, bytraveling along elongate sections, some embodiments may capturewind-power from areas many times the wing-span of the airframes. In someexamples, that area of power extraction is the combined length of theelongate sections multiplied by a wing-span of the airframe. Bycontrast, wind-turbines are limited to harvesting wind-power from anarea of a circle with a radius corresponding to the span of the turbineblades.

Some embodiments described herein may also provide benefits over AWEs.For example, because some embodiments do not use a tether, there are nocosine losses associated with harvesting wind power, nor energy lossesdue to tether drag. Furthermore, some examples may not be classified as“airborne devices,” in contrast to AWEs, and so avoid the disadvantagesof aviation regulations and restrictions.

Returning to FIGS. 1A and 1B, airframes 112 and 116 travel in oppositedirections 120 and 122 on the upper and lower sections 104 and 106,respectively. The airframes change from the upper to the lower sectionalong terminal 108 and from the lower to the upper section on terminal110. Travel along a terminal also causes a change in direction of theairframes. While traveling along a path corresponding to the terminal,the airframe may be yawed to achieve a desired cross-wind orientation.In apparatus 100, an airframe is yawed 180 degrees (in a reference frameon the airframe) when traveling along a path corresponding to theterminal.

In addition, the change in direction results in the airframes travelingwith a first roll to the horizon when traveling on the upper elongatesection 104 and a second roll to the horizon when traveling on the lowerelongate section 106. More specifically, when an airframe in FIGS. 1Aand 1B travels in direction 120, the airframe is rolled 90 degrees tostarboard (in an earth reference frame). When an airframe travels indirection 122, the airframe is rolled 90 degrees to port. By rolling atdifferent angles when traveling in different directions, some examplesof the disclosure allow the airframes to travel crosswind in bothdirections along the elongate sections and orient the airframe at anaerodynamically efficient angle of attack to a relative wind (asdescribed further below).

Electricity may be captured from the motion of the airframes using anyof a number of mechanisms (not shown) or a combination of thosemechanisms. In some examples, a drag on the airframe may be converted toelectricity. This may be achieved using an electric motor or generator,such as, for example, a propeller on the airframe. As the propellerturns, electricity is generated. Other examples may include a carriagewith wheels (see, for example, elements 126 and 128 in FIG. 1) that runalong the track. As the wheels turn, electricity is generated in anelectric motor in the carriage. In some other examples, a cable or otherconveyer belt is connected to the airframe and power is generated at theconveyer hub rather than on-board the airframe. Some examples mayinclude a rack and pinion arrangement, wherein the pinion is attached tothe airframe and the rack is in the elongate section. In furtherexamples, there may be numerous cables, conveyor belts, rails, etc onthe track so that different airframe speeds can be capturedindependently and/or some conveyor belts can be used to accelerate anairframe, when needed.

In some examples, electricity is captured through induction. An electriccoil may be installed in the elongate section and a magnet installed onthe airframe. As the airframe moves, the magnet induces an electriccurrent in the coil which may be captured as electric power. Such anarrangement may beneficially reduce the number of mechanical parts, suchas gears and/or cables. In some examples, the electric coil is woundaround a core. Multiple coils may be used in a single elongate section,so that different airframe speeds can be captured independently and/orso that some coils may be used to accelerate an airframe.

In some examples, the airframes move independently of one another. Thesystem may vary the speed of the airframes on different elongatesections and/or vary the number of airframes on each elongate section atany one time. In certain wind circumstances, it may be beneficial tohave substantially different speeds, for example, to increase the powerextracted from the wind. In some examples, low wind speeds may call fora relatively large number of airframes traveling relatively slowly and,by contrast, high wind speeds may call for a smaller number of airframestraveling relatively quickly. In some examples, a wind direction whichis not perpendicular to the direction of travel of an airframe may callfor different speeds and/or different number of airframes on the tracks.In some examples, the variations in speed and/or number are introducedmanually and in others the variations are introduced automatically. Insome other examples, a combination of manual and automatic controlsintroduce the variations.

To facilitate different speeds and/or different numbers of airframes,the airframes may collect at the terminals when not traveling on theelongate sections. The number of airframes on a section may be analogousto the solidity of a wind-turbine. As used herein, solidity can beunderstood to include a measure of the area that airframes or turbineblades sweep through (swept area) compared to the area occupied by theairframes or turbines blades, respectively. Unlike a wind-turbine,examples of the disclosure can change solidity from one wind conditionto another and from one elongate section to another.

In some examples, airframes with different aerodynamic profiles may beused for different wind conditions. For airframes traveling in adirection aligned with an atmospheric wind, it may be advantageous tomaximize surface area so that more power is captured by the airframe. Onthe other hand, airframes traveling crosswind may benefit from arelatively high lift/drag profile to increase lift (and thereby increasecrosswind speed). Airframes may be stored in a corral and changed inresponse to the wind conditions at a given time.

Such corrals at the end of elongate sections may also be used to changethe ratio of the number of airframes traveling in one direction. Forexample, an apparatus with three or more elongate sections may providefor a different number of airframes traveling in one direction. Such anarrangement may be advantageous when wind conditions favor airframetravel in one direction over the other. For example, when airframestravel in a direction aligned with an atmospheric wind, it may beadvantageous to return the airframes (i.e., in the upwind direction) ata high speed. Resistance (and thus power supplied) to upwind travel maybe reduced by utilizing a low drag profile. By contrast, the downwindspeed may be relatively slow. Accordingly, for three or more elongatesections, it may be beneficial to allocate more elongate sections todownwind travel. The speed of the upwind airframes may be chosen so thatthe airframes are replenished at the upwind corral at the rate needed tosupply the downwind travel.

The corrals may be a sub-section of the terminals, or replace theterminals. The corrals may include auxiliary rails, similar to sidingfor a train. The airframes may be staged in the auxiliary rail andintroduced into the elongate sections as needed. A switch (manual orautomatic) may guide the airframes into the corrals as necessary.

The track may be supported on either end by towers. As used herein, atrack can be understood to include the elongate sections and terminalsthat comprise a closed loop. The towers can take any size or shapesufficient to support the track. The towers may be positioned at the endof each terminal, or may be positioned at inward points on the elongatesection/track. In one example, a number of tracks are arranged side byside, each attached to two towers (similar to power lines attached toutility poles). In this way, multiple tracks can be supported and anindividual track can be taken out of service for maintenance or otherissues, without disturbing the power extraction of other tracks. In someexamples, the elongate sections may be lengthened and supported usingmultiple towers. In these examples, lengthening the sections (ratherthan providing multiple tracks) may increase the efficiency of thesystem because the number of terminals is reduced (thus overall energyloss at terminals is reduced).

In some examples, multiple tracks are stacked on top of each other. Inthese examples, the airframes may travel in contra-rotating directionson track pairs, balancing twisting moments that may be caused by therotation of the airframes on each track and/or the acceleration of theairframes at the terminals.

In some examples, elongate sections and/or whole tracks are arrangedback-to-back. In some examples, tracks are arranged at 90 degrees to oneanother, thereby allowing the airframes to travel crosswind for avariety of incident atmospheric wind directions. In other examples, thetracks are positioned at other relative angles between 0 and 90 degrees,including 15, 30, 45, 60, and 75 degrees.

In some examples, elongate sections are arranged in a lattice. Forexample, rows of elongate sections may run east-west and columns may runnorth-south. When viewed from above, the elongate sections may depict achecker board pattern, with the ground representing the “squares” of thechecker board. This arrangement allows for power extraction regardlessof the orientation of an atmospheric wind. In some examples, theelongate sections may be oriented in multiple, overlaid, equilateraltriangles. This may allow for increased power extraction at a variety ofatmospheric wind orientations and may also allow the elongate sectionsto act as tie-rods. The elongate sections may share towers. Similarly,the elongate sections may share airframes which are transferred betweenelongate section, depending on the orientation of the atmospheric wind.Elongate sections in different orientations may alternate elevation. Forexample, if each north-south and east-west arrangement has four elongatesections, then the north-south arrangements may be positioned at 3, 9,15, and 21 feet, while the east-west arrangements may be positioned at6, 12, 18, and 24 feet. This spacing would allow airframes with wingspanof less than 6 feet to travel on any of the elongate sections withouthitting another elongate section.

In some examples, one or more tracks are moveable relative to the groundor other reference point. For example, one tower associated with thetracks may be moveable and the other fixed so that the track pivotsabout the fixed tower. The moveable tower may be on wheels and connectedto a motor for transport. In this way, some examples may be able to varythe angular offset of the airframes' travel to the incident atmosphericwind direction. Also, some examples with moveable tracks may be able toreorient to a desired angular offset for a variety of atmospheric winddirections.

FIG. 2A illustrates an exemplary airframe 200 according to examples ofthe disclosure. Airframe 200 includes wings 202, fuselage 204, andempennage 210. Empennage 210 includes a vertical stabilizer 212 andhorizontal stabilizers 214.

As will be readily appreciated, airframe 200 is offered as an exampleand numerous variations could be employed without deviating from thescope of this disclosure. For example, some airframes may include only ahorizontal stabilizer or a vertical stabilizer. In some embodiments, anairframe does not include an empennage.

FIG. 2B illustrates an exemplary airfoil 230 according to examples ofthe disclosure. Airfoil 230 may be coupled to a track directly, or maybe part of a larger airframe. In some examples, airfoil 230 may be thecross-section of wings 202 described above with respect to FIG. 2A.

Airfoil 230 depicts an aerodynamically efficient shape providing auseful lift/drag coefficient. Airfoil 230 includes a leading edge 232, atrailing edge 234, suction surface 236, and pressure surface 238. Asrelative wind 240 moves past airfoil 230, the shape induces an upwardforce (as viewed in FIG. 2B) on the airfoil. The force acts on theairfoil in a direction from the pressure side toward the suction side.

As used herein, an airfoil can be understood to be any object, orcross-section of an object, that provides a dynamic force in fluid flow.These include, without limitation, wings, sails, and turbine blades. Anairfoil may be a part of a larger body, with additional components. Forexample, an airframe may include not only an airfoil, but also include afuselage and empennage, such as described above with respect to FIG. 2A.In some examples, the airframe may simply be an airfoil.

When an airfoil is designed with a pressure surface and a suctionsurface, the airfoil may be oriented on a section of a track so that thepressure surface is positioned between the track and the suctionsurface. This arrangement may enable the airfoil to move crosswind atgreater speed.

Chord 242 is an imaginary straight line joining the leading edge 232 andthe trailing edge 234. The airfoil is oriented at angle of attack a(244), which can be understood to be the angle between chord 238 and thedirection of the relative wind 238.

As used herein, the term “relative wind” can be understood to be thevector sum of the created wind velocity and the atmospheric windvelocity. FIG. 2C illustrates an exemplary relative wind velocity 254 inaccordance with one example of an object (not shown) moving crosswind.Vector 250 represents the ground velocity of the object and vector 252represents the atmospheric wind velocity.

For purposes of explaining the relative wind velocity 254, the “createdwind” must be first understood. The created wind velocity is simply themagnitude of the ground velocity 250, but reversed in direction. Tocalculate the relative wind velocity, the created wind velocity andatmospheric wind velocity 252 are vector-summed to result in therelative wind velocity 254.

Returning to FIG. 2B, various angles of attack may be employed. In someexamples, the angle of attack is calculated as a function of a desiredlift/drag ratio. For example, if a predetermined lift/drag requires a 6degree angle of attack, then the pitch of the aircraft will be alteredbased on the atmospheric wind and ground speed. If the atmospheric windspeed is 8 m/s and the ground speed is 50 m/s, then the relative windspeed will be at 9.09 degrees (tan⁻¹ (8/50)) to the track. To achieve anangle of attack to the relative wind of 6 degrees, the airfoil will beangled at −3 degrees to the track.

In general, a pitch angle relative to the track can be determined withthe atmospheric speed, the ground speed, and a desired angle of attack.First, the angle of the relative wind to the track is determined fromatmospheric wind speed and the ground speed. The angle of the relativewind is then subtracted from the desired angle of attack, to result inthe desired pitch angle of the airframe relative to the track.

The angle of attack may be varied for different wind conditions. Thatis, considerations such as efficient power extraction and limitedstructural integrity may determine a desired angle of attack. The angleof attack may be controlled via the empennage (i.e., via elevators onthe horizontal stabilizers), or mechanically by moving/reorienting thecoupling between the track and airframe.

In some examples, the angle of attack need not be the same whentraveling in different directions on elongate sections. This may accountfor an atmospheric wind that is not perpendicular to the elongatesections.

An empennage may be used to implement a gust safety factor, passivelyand/or automatically. A gust increases the angle of attack. The tail maybe designed so that a gust increases the lift of the tail. When the tail“lifts” (relative to the wind), the angle of attack of the airframedecreases, and the airframe's attitude returns to within a predeterminedrange.

In some examples, the airframe can be selectively oriented to havelittle or no force generation. This may be beneficial for addressingexcessive winds or misdirected winds, and for downtime such asmaintenance operations. By setting roll angle to zero (relative to thehorizon) and pitch angle to zero (or zero lift Angle of Attack), andallowing the airframe to freely yaw, the airframe will exert littleforce on a track.

In some examples, an electricity extraction apparatus is anchored usingone or more bridles. FIGS. 3A and 3B illustrate an exemplary bridlingsystem according to examples of the disclosure. FIG. 3A illustrates aside view of bridling system 300 and FIG. 3B illustrates a top view.

Bridling system 300 includes bridles 308 and 310 coupled to upperelongate section 302 and lower elongate section 304, respectively. Insome examples, elongate sections 302 and 304 may be the upper and lowerelongate sections described above with respect to FIGS. 1A and 1B.

Bridles 308 and 310 are co-anchored at anchor point 306. In someexamples, two bridles may be anchored at different anchor points.Bridles 308 and 310 define angles γ₁ and γ₂ with respect to a referenceline containing anchor point 306. In some examples, the reference linemay be the level of the ground, the horizon, or other physicalreference.

FIG. 3B illustrates the bridling system from above, depicting multiplebridles attached to each elongate section. Elongate section 302 is theonly elongate section visible in FIG. 3B due to the vertical alignmentof the elongate sections in this example.

As can be seen in FIG. 3B, bridling system 300 includes bridles 308,314, and 318 distributed across the elongate section. Each bridle isattached to the elongate section 302 at anchor points 306, 312, and 316respectively. The plurality of bridles divides the length 324 of theelongate section 302 into sub-lengths 320 and 322.

As noted above, elongate section 302 is obscuring elongate section 304because of the vantage point of FIG. 3B. For the same reason, only upperbridles 308, 314, and 318 are viewable in FIG. 3B. System 300 alsoincludes at least lower bridle 310 obscured by upper bridle 308 and mayinclude additional bridles obscured by upper bridles 314 and 318.

The bridling systems of this disclosure may beneficially distribute thedownwind forces, reducing their destructive effect on the powerextraction apparatus. In a wind-turbine, all forces are concentrated atthe hub (or central point), which causes large moments on the tower. Forat least this reason, the tower in a wind-turbine may be very large. Inthe apparatuses described herein, the downwind forces are distributed onthe multiple airframes. Further, the bridling distributes the downwindforces on the sub-lengths so that any one sub-length experiences asubstantially reduced moment.

Although sub-lengths 320 and 322 are approximately equal in FIG. 3B,other examples may include different sub-lengths. Also, although threebridles are illustrated in FIG. 3B, other examples may include anynumber of bridles.

Each sub-length may be chosen to account for a variety ofconsiderations. For example, the wing area of the airframe, the speed ofthe airframe, strength of the rail, etc. In some examples, thesub-lengths are determined as a function of one or more of the densityof the fluid, area of the airframe, the speed of the carrier on thetrack, airframe lift coefficient, and the modulus of elasticity of thetrack. In some examples, the sub-lengths are approximately 2.5 meterseach. In some examples, the number of bridles is determined by thenumber of airframes traveling on an elongate section at one time and maybe chosen so that there are more sub-lengths than traveling airframes atone time.

In some examples, a bridling system is chosen to suit a surroundinglandscape and specific implementation factors. In some examples, eachbridle is anchored individually and runs straight from the ground to thetrack. In other examples, the bridle is configured similarly to acable-stayed bridge: the bridles attach to one (or more) anchor points,and then fan out to the rail attachment points. In another example, thebridles are configured similarly to a suspension bridge: a suspensioncable forms an arc between two attachment points and the individualbridles are attached to this arc. In yet another example, the bridlesystem includes a flying anchor point. In this example, a guywire/tether goes up from the ground to a central “flying anchor.”Bridles attach to this “flying anchor” and fan out to the track. Inanother example, a helper pylon is installed between the main anchorpoint and the track. A guy wire then goes from the ground to the helperpylon at a large angle, and then is redirected (via individual bridlesor grouped anchors) to the track. By redirecting the bridle, the helperpylon absorbs some of the downward forces, allowing total bridle lengthto be shorter (or effective angle to be lower) thereby reducing cosinelosses due to elevation.

In some examples, the bridles may have different lengths for uppersections and lower sections. This may cause the track to be rolled at adesired roll angle, which then may force the airframe to a desired rollangle (see discussion of roll angle below).

In some embodiments, an upper elongate section is rigidly attached to alower elongate section at bridle attachment points. Again, differentialbridle lengths may be used. This arrangement may also force the elongatesections to the proper angle. In comparison to the examples in theprevious paragraph, this configuration accentuates line lengths slightly(by increasing separation), allowing for more practical control.

Some examples may include dual direction flying buttress when variousatmospheric wind directions are to be harvested. In these examples, anupside down V may be placed between “upwind” and “downwind” sections.The sections are rigidly attached to this buttress, and thecarriers/airframes are attached to the sections such that they can onlyslide. Two upwind bridles and two downwind bridles are attached to thebuttress in such a way that they may avoid all potential orientations ofthe airframe. In this way, it is possible to construct a dual directionapparatus which also maintains the rails at the proper roll angle.

Bridling may introduce additional forces on the track. For example, FIG.3C illustrates the forces on power extraction system 350 when airframe360 is rolled 90 degrees to the horizontal according to examples of thedisclosure. Airframe 360 produces forces in the crosswind direction (notshown) which is harvested as power. Airframe 360 is coupled to elongatesection 352 via carrier 358 and produces downwind force (F_(D)) onelongate section 352.

The downwind force (F_(D)) is balanced by bridle 354. However, bridle354 is oriented at angle γ and so the bridle force (F_(B)) acts at angleof 90−γ from the horizontal. As a result, the bridle force (F_(B)) hasboth a horizontal component (for balancing the downwind force of theairframe) and a vertical component. This vertical component, which hasno counter balance in the example of FIG. 3C, tends to pull the trackdownwards, which may put stress on the track.

In some embodiments, a roll is introduced into the airframe that is lessthan 90 degrees from the horizontal. The roll may orient the airframewith respect to the rail and bridle so that the forces acting on theairframe are approximately in the direction of the bridle.

FIG. 3D illustrates a side view of an exemplary roll introduced onairframe 360 according to examples of the disclosure. As above, bridle354 is coupled to a ground surface at anchor point 356 and bridle 354 isangled at γ to the horizontal. To balance the bridle force, airframe 360is rolled at angle φ to the horizontal (note that the horizontal isreferred to here, but any reference line could be used).

In the example of FIG. 3D, airframe 360 is rolled at angle φ=90−γ. Insome embodiments, γ may not equal 90−γ, but is rolled a number ofdegrees away from 90−γ. In some examples, a maximum degree offset from90−γ may be calculated by a limit on efficiency losses. In some furtherexamples, an algorithm may be used to relate the degree offset (γ) tothe limit on efficiency loss (EL). Exemplary algorithms may includeEL=(1−cos γ). For a given limit on efficiency loss, the maximum offsetangle can be determined. The maximum degree offset may also be limitedby the structure of the track. As the angle between the airframe rolland bridle elevation increases, the stress on the track increases. For agiven material, a maximum offset may be calculated to keep stress on thetrack within a predetermined limit.

Airframe 360 is coupled to elongate section 352 via carrier 358. Carrier358 may be rotatable with respect to elongate section 352 so thatairframe 360 can change its angle with respect to elongate section 352.Some carriers may include a servo to orient the airframe to the desiredroll angle, as discussed in more detail below.

In some examples, an upper elongate section and lower elongate sectionmay have different bridle angles (see FIG. 3A) and thus different rollson the airframes. As the airframe travels along the terminal, a changein roll must be introduced so that the airframe can assume the correctroll angle for the elongate section. This may be achieved by yawing theairframe at 180 degrees while also rolling the airframe a desired amountas the airframe travels along the path corresponding to the terminal. Insome variations, the magnitude of this roll will be the sum of thebridle angles.

In some embodiments, the roll of the airframe may be measured andadjusted accordingly. In some examples, the roll may be measured usingsensors attached to the airframe, or via an inertial navigation system(such as a gyroscope, accelerometer, and gps) integrated via a sensorfusion algorithm such as a kalman filter. The airframe's position mayalso be sensed via RFID or other near field communications, with thoseoutputs utilized in the sensor fusion algorithm. In some embodiments,the roll of the airframe is measured using mechanical sensors attachedto the carriage.

There are a number of mechanisms by which the roll of the airframe canbe controlled. In some examples, the airframe is equipped with aileronson its wings. By manipulating the ailerons, the roll of the airframe canbe controlled. In some embodiments, the elongate section may introduce anatural roll to the airframe. For example, an elongate section may beconstructed of flexible material that naturally flexes to roll theairframe and balance the bridle forces. In some embodiments, a track isrotatably attached to supporting towers. In this way, the track maynaturally reorient to an angle to the horizontal that corresponds to anappropriate roll in the airframe. In some examples, the airframeattachment to the carrier may be offset laterally from a center ofpressure to induce a roll. When the desired roll is achieved, theattachment may be reset to the center of pressure. Some embodimentscontrol roll by tethering the wing tips differentially to the carrier.For example, a starboard tether may be lengthened and a port tethershortened, resulting in a roll toward port.

Although the above description focused primarily on balancing bridleforces with the aerodynamic forces on the airframe when determining rollangle, other forces may also be considered. These forces may include theforce of gravity on the airframe and carrier, the force of gravity onthe elongate section, the force of gravity on support towers andbuttresses, the force of drag on the elongate section, and any buoyancyof connected objects (such as in a water embodiment or airborneembodiment). In some embodiments, the elongate section isaerodynamically shaped to reduce drag.

In some airborne examples, the gravitational force induced by the weightof the apparatus may be balanced by a lift induced by the airframes.Such an apparatus may first be raised to a desired altitude using acrane, aircraft, or buoyancy device, for example. Once at theappropriate altitude, wind may be utilized to change or maintain thealtitude. This may beneficially allow airborne embodiments to capturehigh velocity wind vectors at high altitudes.

In some examples, multiple bridles may be attached to a single point onthe track and “fan out” so that each bridle creates a different angle tothe track (when viewed from above). This arrangement may beneficiallyoffset aerodynamic forces for a variety of relative wind directions.

In some examples, the angles between the track and the outer-mostbridles and the angles between bridle pairs may be equal. In furtherexamples, the equal angles may each be 180 degrees divided by one plusthe number of bridles (e.g., 180/(n+1), where n is the number ofbridles). For example, for two bridles attached to the same point, theangle between the bridles may be 60 degrees and the angle between eachbridle and the track may be 60 degrees. In other examples, the anglesbetween the track and the outer-most bridles and the angles betweenbridle pairs may not be equal.

By attaching multiple-bridles to the same point on the track, andarranging the bridles at different angles to the track, some examplesmay improve aerodynamic force offset for various atmospheric wind speedsand airframe wind speeds. For example, if an airframe is travelingrelatively quickly on the track, then the relative wind may beapproximately parallel to the orientation of an elongate section(resulting in the force being perpendicular to the elongate section). Insuch a scenario, multiple bridles connected to a single point, and atdifferent angles, on the elongate section will allow the force to bedistributed between the bridles.

By contrast, if the airframe speed is slower, the relative wind may notbe parallel to the elongate section (and so the force may not beperpendicular to the elongate section). In such a scenario, theaerodynamic force may be more aligned with one of the bridles and thatbridle can offset the dynamic force. Compare this to a single bridleembodiment, where a non-perpendicular force (to the elongate section) isonly partly balanced by the bridle.

Multiple bridles at different angles may allow for optimization acrossvarious wind conditions. For example a relatively slow atmospheric windmay call for high airframe speed and low angle of attack in an “upwind”direction, but a low airframe speed and high angle of attack in a“downwind” direction.

Although the above description of bridling and induced rolling focusesprimarily on systems with two elongate sections, one of ordinary skillin the art will readily appreciate that the concepts can be applied tosystems with a single elongate section or systems with multiple elongatesections. One of ordinary skill in the art will also readily appreciatethat the bridling and induced roll concepts described above can beapplied to non-elongate sections. The description above applies to anyarrangement of a system including a bridle and an airframe, where theairframe can be rolled to offset forces in the bridle.

Turning now to the terminals that connect the elongate sections, it willbe appreciated that the airframe exerts a centrifugal force on the trackwhile changing direction. This centrifugal force may requirereinforcement of the track at the terminals.

In addition to the centrifugal forces at the terminals, aerodynamicforces are also introduced. Specifically, the airframe may yaw at theterminal. This will result in the outer wing tip having a higher groundspeed then the inner wing tip. Thus, the outer wing tip has a differentangle of attack. Therefore, the apparent angle of attack and apparentwindspeed are different on the inner side and the outer side. This mayresult in more lift on the outer wing than the inner wing.

In some embodiments, the forces and positioning needs at the terminalsare addressed by reducing the absolute velocity of the airframe at theterminals. This deceleration can be harvested as power.

In some embodiments, the airframe is rolled at the terminals to counterforces created by the yaw. For example, if the outer ailerons are raisedand the inner ailerons are lowered, the outer lift will be decreased andthe inner lift increased. This may serve to even out the liftdistribution and reduce the rolling moment which must be absorbed by thestructure.

Although the description of embodiments above was offered primarily withrespect to tracks anchored to ground, the disclosure is not so limited.In some examples, the track may be airborne or may be offshore.

In addition, although single wing airframes were primarily discussedabove, some embodiments may include multi-wing aircrafts such asbi-planes or other multiplane airframes. The wings may be stacked orarranged one behind another, or a combination of both.

FIG. 4 illustrates method 400 of extracting power according to examplesof the disclosure. Method 400 includes providing a track 402. The trackmay include a first elongate section and a second elongate section lowerthan the first elongate section. Method 400 includes coupling anairframe to the track 404 so that the airframe travels crosswind to anatmospheric wind.

In some examples, the method of extracting power may also includeattaching a bridle to the first elongate section. The bridle may beanchored and angled at γ degrees to a horizon. The method may alsoinclude rolling the airframe at approximately 90−γ degrees to thehorizon when coupled to the first elongate section. Some examples mayinclude more than one bridle.

In some examples, the method may also include rolling the airframe at afirst angle to a horizon when the airframe is coupled to the firstelongate section and rolling the airframe at a second angle to thehorizon when the airframe is coupled to the second elongate section, andwherein the first angle is different from the second angle.

Some methods may include coupling a terminal between the first andsecond elongate sections, wherein the airframe decelerates when theairframe transitions from the first elongate section to the terminal,and wherein the airframe accelerates when the airframe transitions fromthe terminal to the second elongate section. In some examples, a methodmay include coupling a terminal between the first and second elongatesections and yawing the airframe as it travels along the terminal.

As noted above, the disclosure is not limited to wind-power. Someexamples may include other gases or fluids. Exemplary hydropowerembodiments may include a river installation or a tidal powerinstallation. In some other examples, the electricity extractionapparatus may be attached to buoyant devices, which may create lift. Bymanipulation of roll angle (either through structure or activecontrols), the apparatus can be maintained at a desired depth or heightto increase energy capture, for example. When used herein, terms thatmay suggest a specific application (such as crosswind and atmosphericwind) should be understood to have analogous terms in other fluid flows.

Further, as used herein, the term “elongate section” may be understoodto be any structure to which an airframe can be coupled and travelcrosswind for distances many times the size of the airframe. An elongatesection may not necessarily be linear and may include curves or othernon-linear aspects. In some embodiments, an apparatus or method forextracting power may include a single elongate section or multipleelongate sections arranged horizontally, rather than the verticalorientation described herein.

Although the disclosed embodiments have been fully described withreference to the accompanying drawings, it is to be noted that variouschanges and modifications will become apparent to those skilled in theart. Such changes and modifications are to be understood as beingincluded within the scope of the disclosed embodiments as defined by theappended claims.

What is claimed is:
 1. An apparatus for extracting power comprising: atrack comprising first and second elongate sections, wherein the firstelongate section is positioned above the second elongate section; anairfoil comprising a suction surface and a pressure surface, wherein theairfoil is coupled to the track so that the pressure surface liesbetween the suction surface and the track, and wherein the airfoil ismoveable in opposite directions when alternately coupled to the firstelongate section and second elongate section; a bridle coupled at afirst end to the first elongate section and at a second end to ananchor, wherein the bridles is angled at γ degrees to a horizon and theairfoil is rolled at approximately 90−γ degrees to the horizon whencoupled to the first elongate section; and a power generator to harvestpower from an atmospheric wind through the movement of the airfoil. 2.The apparatus of claim 1, further comprising at least three bridlescoupled to the first elongate section.
 3. The apparatus of claim 1,wherein the airfoil has a first roll to a horizon when the airfoil iscoupled to the first elongate section and a second roll to the horizonwhen the airfoil is coupled to the second elongate section, and whereinthe first roll is different from the second roll.
 4. The apparatus ofclaim 1, wherein the track further comprises a terminal connecting thefirst and second elongate sections, wherein the airfoil decelerates whenthe airfoil transitions from the first elongate section to the terminal,and wherein the airfoil accelerates when the airfoil transitions fromthe terminal to the second elongate section.
 5. The apparatus of claim1, wherein the track further comprises a terminal connecting the firstand second elongate sections, wherein the airfoil yaws as it travelsalong the terminal.
 6. A method of extracting power comprising:providing a track comprising a first elongate section and a secondelongate section lower than the first elongate section; coupling anairframe to the track, wherein the airframe comprises a suction surfaceand a pressure surface, wherein the airframe is coupled to the track sothat the pressure surface lies between the suction surface and thetrack; positioning the track so that the airframe travels crosswind toan atmospheric wind; attaching a bridle to the first elongate section;anchoring the bridle, wherein the bridle is angled at γ degrees to ahorizon; rolling the airframe at approximately 90−γ degrees to thehorizon when coupled to the first elongate section; and harvesting powerfrom the atmospheric wind through the movement of the airframe.
 7. Themethod of claim 6, further comprising attaching at least three bridlesto the first elongate section.
 8. The method of claim 6, furthercomprising rolling the airframe at a first angle to a horizon when theairframe is coupled to the first elongate section and rolling theairframe at a second angle to the horizon when the airframe is coupledto the second elongate section, and wherein the first angle is differentfrom the second angle.
 9. The method of claim 6, further comprisingcoupling a terminal between the first and second elongate sections,wherein the airframe decelerates when the airframe transitions from thefirst elongate section to the terminal, and wherein the airframeaccelerates when the airframe transitions from the terminal to thesecond elongate section.
 10. The method of claim 6, further comprisingcoupling a terminal between the first and second elongate sections andyawing the airframe as it travels along the terminal.